Field of the Invention
The invention concerns a method, a magnetic resonance apparatus, and an electronically readable data storage medium for accelerated acquisition of magnetic resonance data.
Description of the Prior Art
Magnetic resonance (MR) is a well-known technique for producing images of the interior of an object. Expressed simply, the object to be imaged is positioned in a magnetic resonance scanner in a strong, static, homogeneous basic magnetic field, also known as a B0 field, having field strengths of 0.2 to 7 tesla or more, so that the object's nuclear spins align along the main magnetic field. To trigger nuclear spin resonances, radiofrequency (RF) pulses are radiated into the object e.g. for excitation or refocusing signals by the nuclear spin resonances that are triggered are measured as so-called k-space data and MR images are reconstructed or spectroscopic data obtained on the basis thereof. For spatially encoding the scan data, rapidly switched magnetic gradient fields are superimposed on the basic magnetic field. The scan data obtained are stored in a k-space matrix as complex numerical values. An associated MR image can be reconstructed from the populated k-space matrix e.g. by a multidimensional Fourier transform.
Most of the methods used in magnetic resonance systems are based on measuring echo signals produced by RF refocusing pulses (spin echo (SE) methods) or by switching echo signals produced by gradients (gradient echo (GRE) methods). The aforementioned also include methods that generate a so-called “stimulated echo” by radiation of at least three RF pulses and are therefore also known as stimulated echo methods. However, one of the disadvantages of the latter is that they require longer scan times than e.g. GRE methods or other SE methods due to the three RF pulses having to be radiated in a mutually synchronized manner.
Methods aimed at shortening the scan times include so-called parallel imaging whereby, in an EPI (echo-planar imaging) scheme, for example, the echo-planar readout train can be shortened, which not only achieves a reduction in distortion but also enables the repetition time (and therefore ultimately the scan time) to be reduced, but at the cost of simultaneously reducing the signal-to-noise-ratio (SNR). However, particularly for scans using stimulated echoes, the readout train accounts for a rather small proportion of the scan time, which means that only a small amount of scan time can be saved by parallel imaging.
Methods for reducing the scan time are also known in which several slices are excited simultaneously and the signal superimpositions in the resulting images are separated by spatial sensitivity profiles of the reception coils. Such methods are termed e.g. multiband, simultaneous multislice, or slice multiplexing techniques. An example of a simultaneous multislice method of this kind is described e.g. in the article “Accelerated Human Cardiac Diffusion Tensor Imaging Using Simultaneous Multislice Imaging”, MRM 73, p. 995 (2015) by Lau et al.
Particularly in connection with MR methods that use inversion pulses to suppress unwanted signal components, such as FLAIR (fluid attenuated inversion recovery) or STIR (short TI inversion recovery), for example, methods for the interleaved application of inversion modules and acquisition modules of different slices are also known that likewise provide a reduction in scan time. Such a method is known, for example, from the article “Whole-Body STIR Diffusion-Weighted MRI in One Third of the Time”, Proc. Intl. Soc. Mag. Reson. Med. 21, p. 2059 (2013) by Stemmer et al.
Using magnetic resonance techniques, specially encoded signals can also be generated and measured, allowing particular issues to be investigated. For example, MR diffusion imaging can be used to investigate issues e.g. in stroke diagnostics or planning prior to brain surgery or even for diagnosing tumors in the trunk of the body. Although spin echo based echo-planar imaging methods are used here, because they have a very good SNR per unit time, there are cases in which stimulated echo based scans are advantageous. These include, for example, examining tissue types that have very short transverse relaxation times (T2), such as muscle tissue, but also scans involving ultra-high magnetic fields (e.g. 7T or higher) for which the transversal relaxation time is generally reduced, as described e.g. in the article “Diffusion Weighted Imaging at 7T with STEAM-EPI and GRAPPA”, Proc. ISMRM 18, p. 3994 (2010) by Dhital et al. Another case is for scans in which long diffusion times are deliberately examined in order to be able, for example, to analyze spatially limited movement processes. A method such as the last mentioned is described, for example, in the article “Time-dependent diffusion in skeletal muscle with the random permeable barrier model (RPBM): application to normal controls and chronic exertional compartment syndrome patients”, NMR Biomed. 27, p. 519 (2014) by Sigmund et al.
There is therefore increased interest in methods based on stimulated echoes.
FIG. 1 schematically illustrates a pulse sequence diagram of the kind used in the prior art for obtaining a diffusion-weighted stimulated echo image. Shown in the top line “RF” are the RF pulses that are radiated, in the second line “Gs” the gradients to be switched in the slice selection direction, in the third line “Gr” the gradients to be switched in the readout direction, and in the last line “Gp” the gradients to be switched in the phase encoding direction. The breaks in the right-pointing arrows indicate that the duration shown is represented in truncated form. With the excitation pulse Rf1, a slice selection gradient SG is switched to limit the effect of the RF pulse RF1 on a particular slice in the imaging volume. Instead of limiting to a slice, limiting to a slab is similarly possible. After the excitation pulse RF1, diffusion encoding by encoding gradients KG takes place, followed by a second “storage” RF pulse, RF2, which again acts only on the same slice as the first RF pulse hf1 as a result of the slice selection gradient SG. This second RF pulse RF2 “stores” part of the data encoded in the generated magnetization in the form of longitudinal magnetization which decays only with longitudinal relaxation T1 in the following evolution time (i.e. much more slowly than with the transverse relaxation T2). Such so-called modulated longitudinal magnetization is described in more detail e.g. in the article “Extended Phase Graphs: Dephasing, RF Pulses, and Echoes—Pure and Simple”, JMRI 41, p. 266 (2015) by Weigel. After a defined delay (mixing time TM), again with simultaneous switching of a slice selection gradient SG, a third “restoration” RF pulse RF3 restores the stored coherence to the transverse plane where, after diffusion decoding by switching of the decoding gradients DG, e.g. spatially-encoded scan data are acquired (e.g. using an echo-planar readout train GR). The purpose of the encoding and decoding gradients KG and DG is to produce a defined diffusion contrast (e.g. a particular b-value along a particular direction).
Especially when long evolution times are desired in such a stimulated echo method, and therefore the mixing time TM is selected correspondingly long, e.g. TM>100 ms, it becomes clear that a large proportion of the scan time is formed by the mixing time intervals TM.